1. Field of the Invention
The present invention relates generally to devices for measuring volumetric fluid flow and more specifically to travelling wave or flexible membrane fluid flow meters which detect and measure the flow of compressible and non-compressible fluids in response to the undulations of a flexible membrane in a fluid flow passage.
2. Description of the Prior Art
Flexible membrane fluid flow meters operate by measuring the undulation of a flexible membrane caused by fluid flow past the membrane in a flow chamber. In a previous design for a flexible membrane fluid flow meter, as shown for example in Japanese Pat. 1,455,80 published Jan. 22, 1988 for Ohbaru Machine Industry Co., Ltd, a flexible member is mounted across a clamping span in a flow passage at single point clamping points by affixing each membrane end for free rotation about a rod mounted in the housing forming the passage. The length of the active flexible membrane between the single point clamping points is selected to be greater than the span between the clamping points so that membrane is free to undulate in the fluid flow. The excess membrane length, that is, the excess of the active membrane length between the clamping points over the span between the clamping points, substantially effects the membrane behavior.
Under ideal conditions, the excess membrane length should be sufficient to permit two or three contact points between the active membrane and the flow passage so that discrete quanta of determinable volumes of fluid are indicated by undulations of the active membrane. The excess membrane length for a particular flow passage cross-section and clamping span is a function of fluid flow rate. In order to accommodate wider ranges of measurable flow rates, conventional flexible membrane flow meters such as the Ohbaru design referenced above, proposed embodiments in which the active ends of the flexible membrane were clamped within rollers so that the excess membrane length could be adjusted for different flow rates. Clamping the active membrane ends within rollers provides single point clamping in that relatively unrestricted active membrane undulation is permitted within the clamping span in the same general manner as affixing the active membrane ends for free rotation about mounting rods.
One substantial problem with such conventional flexible membrane flow meters involved short operating life resulting from membrane fatigue due to repetitive flexing at the upstream and downstream clamping points and/or loss of membrane resiliency from remaining in the same curved shape for an extended period of time. Clamping the active membrane ends between rollers permitted the convenient renewal or replacement of the active membrane by unwinding unused flexible membrane from a storage reel through one set of clamping rollers, across the clamping span and through the other set of clamping rollers onto a take up reel. The storage and take up reels were adjusted to leave the appropriate excess membrane length within the clamping span for the flow rate to be measured.
An improved flexible membrane fluid flow meter is shown in U.S. Pat. No. 4,920,794, issued May 1, 1990 to the inventor hereof the disclosure of which is incorporated herein by this reference, in which the active portion of the flexible membrane extended between guide blocks fixed to the passageway or housing. The flexible membrane extended through the guide blocks to spring mounted rods which dynamically adjusted membrane excess length in response to fluid flow rates.
Active membrane behavior within the clamping span was enhanced by outwardly curved guiding surfaces having configurations similar to the flexible membrane in either of its two respective positions taken up just before and after release of a quantum of fluid. These guide surfaces included extensions from the clamping points of the guide blocks into the clamping span to the surface of the fluid passageway. Such extensions were made, for example, from permeable materials such as wire mesh so that they would not interfere with fluid flow. These guide extensions were provided to induce a degree of instability in the behavior of the flexible membrane to cause, even under a relatively low kinetic energy input, a relatively fast change of position of the active membrane between the positions taken up thereby just before and after passage of a quantum of fluid.
Enhancements of active membrane behavior were also achieved by applying resilient materials to the outwardly curving guiding surfaces, and even by resiliently mounting the guide blocks to the flow passage with springs. The flexible membrane was provided with thickened ends, and/or varying cross-sectional thickness along its length, to reduce membrane failure due to fatigue stresses.
A further improved flexible membrane fluid flow meter is shown in U.S. Pat. No. 5,069,067, issued Dec. 3, 1991 to the inventor hereof the disclosure of which is incorporated herein by this reference, in which active membrane behavior was enhanced with multi-component membranes. Stopper members in the form of flexible leaf springs further enhance active membrane behavior by acting as flexible extension guides extending outward from the inner surfaces of the guide blocks. The stopper members limit the flexing motion of the active membrane which strikes the stopper members sequentially as it flexes causing piezoelectric elements mounted thereon to sequentially generate signal pulses representing the passage of a quantum of fluid flow.
In addition, excess membrane length within the clamping span was dynamically adjusted in accordance with fluid flow rates by diaphragms and drogues acted upon by the flowing fluid to reposition the spring mounted membrane mounting rods. In particular, during periods of minimal flow rates, the membrane could be maintained in a fully flattened condition by springs acting upon the mounting rods. In addition, the distance between the spring mounting clamps at the ends of the membrane was adaptively changed to compensate for higher flow rates.
Still further improvements in flexible membrane fluid flow meter design are required, however, to provide additional control of membrane behavior within the clamping span in order to increase the useful range of fluid flow rates and fluid characteristics that may be accurately measured, to further improve membrane operating lifetimes, and to reduce the cost and complexity of meter construction and operation.